There has been used an apparatus for the control of an access to a video memory in which an access is allocated from a CPU to a video memory during the first half of four bits in a horizontal one character cycle of eight bits, and an access is allocated to a character generator of the video memory during the latter half of four bits therein. In the apparatus for the control of an access to a video memory, flickers are prevented from being occured on a screen because the writing and reading of data which are performed from the CPU to the video memory and the access to the character generator of the video memory are divided sequentially.
According to the apparatus for the control of an access to a video memory, however, there is a disadvantage that an access of the video memory can not be controlled in its timing because the access timing is fixed as mentioned before. In a personal computer in which a processing time is widely varied, for instance, from 40 ns to 139 ns dependent on a resolution of the screen, therefore, a property of the video memory is not sufficiently utilized even if the video memory is a high speed memory. On the other hand, a low speed memory can not be used in an apparatus in which a high speed memory is not required in view of a specified characteristic in a case where an access timing is fixed in a high speed mode.
There is a further disadvantage that a throughput of the CPU is decreased because the CPU has to wait the writing of data into the video memory and reading of data therefrom during a display cycle of the video memory, although flickers are prevented from being occured on the screen.